1. Field of the Invention
The present invention relates to a LED package structure, and particularly relates to a LED package structure adapted for lead-free solder technology in order to prevent a short circuit.
2. Description of Related Art
LEDs (Light Emitted Devices) are broadly applied to various electronic products. The LEDs are small and highly reliable, and can be mass produced and adapted for various displays, such as an indication light with low efficiency or outdoor advertising and traffic lights with high efficiency. In comparison with conventional lights, the LEDs have no filaments, have no toxic materials (mercury), have a low power consumption, and have a long service life. A conventional method for packaging the LEDs includes following steps. A plurality of metallic frames, connected one by one at intervals, are punched. A silver layer is electro-plated on the metallic frames, and LED dies are disposed on the frames as light sources. An anode contact and a cathode contact are formed on a respective one of the metallic frames by connecting at least two wires between the frame and the respective LED die. After that, the LED die with the frame is covered with epoxy as a whole transparent package. The European Union has adopted a directive (2002/95/EC RoHs announced) to restrict the use of certain hazardous substances in electrical and electronic equipments starting in Jul. 1, 2006, to minimize their threat to human health. The hazardous substances include lead, cadmium and mercury, and ploy-brominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) using the same. As a result, every country has started to institute bans on these materials, and lead-free alloy is a basic requirement for electronic products. For information and electronic industries, the ban on hazardous substances has a huge influence on manufacturing, such as using solder without lead, guiding the lead-free solder technology into the processes for PCB and the contacts of components, and considering the lead-free joints inside the components. The conventional proportion for tin-lead solder is usually 63:37, and the melting point of this alloy is about 183° C. In contrast, the new lead-free solder has corresponding temperature characteristics, such as pure tin solder (Sn), tin solder with silver and copper (SnAgCu), tin solder with copper (SnCu) or tin solder with silver (SnAg), and has a melting point higher than that of the conventional solder. Not only must the relative profile of the heating process be modified corresponding to the new solder, but also the standards for tests and verifications must be altered to meet the environmental requirements. The most important point is that the wetting velocity of the lead-free solder is much higher than that of the conventional tin-lead solder, so that both the heating profile during the processes and the structure design of the component must be modified to overcome the shortages. The conventional package structure of the LED is given as an example, and is illustrated in FIG. 1. The LED has first and second contacts 1a and 2a separated from each other. A LED die 3a is disposed on the first contact 1a. Two wires 4a and 5a are connected between the LED die 3a, the first contact 1a and the second contact 2a. A package body 7a encloses the LED die 3a, and the first and the second contacts 1a and 2a, at the same time. If there is any seam or crack between the package body 7a and the first contact 1a or between the package body 7a and the second contact 2a, the lead-free solder permeates the package body 7a due to the too-high wetting velocity thereof. A short circuit or a failure mode will occur when the lead-free solder contacts the LED die 3a, or when the first and the second contacts 1a and 2a conducted with each other via the lead-free solder.